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We use angle resolved photoemission spectroscopy (ARPES) to study the momentum dependence of the superconducting gap in NdFeAsO0.9F0.1 single crystals. We find that the Γ hole pocket is fully gapped below the superconducting transition temperature. The value of the superconducting gap is 15 ± 1.5 meV and its anisotropy around the hole pocket is smaller than 20% of this value -consistent with an isotropic or anisotropic s-wave symmetry of the order parameter. This is a significant departure from the situation in the cuprates, pointing to the possibility that the superconductivity in the iron arsenic based system arises from a different mechanism.PACS numbers: 74.25.Jb, The gap function is the single most important quantity that can be used to reveal the pairing mechanism of a superconductor. It's symmetry and shape in momentum space are intimately linked to the many body interactions that are responsible for the creation of the Cooper pairs. The recent discovery of superconductivity in iron arsenic based materials [1,2,3,4] has initiated intense experimental [5,6,7,8,9,10,11,12,13,14,15] and theoretical [16,17,18,19,20, 21] effort. The undoped, non-superconducting systems of both oxygen containing RFeAsO (R=La, Nd, Sm) and oxygen free AFe 2 As 2 (A=Ba, Sr, Ca), display structural [1,22,23,24,25] and magnetic [6,26] phase transitions at elevated temperatures. Doping with RFeAsO with fluorine (electron doping) or AFe 2 As 2 with potassium (hole doping) leads to a suppression of the transition temperature and the emergence of superconductivity [1,2,3,4,23]. Perhaps most remarkably, it has recently been discovered [27] that undoped CaFe 2 As 2 can also be made superconducting by applying a very modest amount of external pressure ∼ 5 kbar. One of the most pressing questions is whether the mechanism of the superconductivity in this system is similar to that in the classical low temperature superconductors or the cuprate high temperature superconductors, or if this is a completely new route to the superconducting state. The large atomic masses of iron and arsenic, combined with the very high critical temperature, seem to exclude conventional phonon mediated pairing. It should be pointed out again, though the superconducting phase transition appears to be in close proximity to a suppressed structural phase transition. This immediately brings the possibility of phonons and their role in Cooper pair formation back to the forefront. The fact that in pure CaFe2As2 this can be seen as function of modest pressures, raises the possible role of phonons even further, even though the lack of light elements in the common FeAs-layers requires a subtly enhanced electron-phonon coupling between Fe-3d electrons and vibrations of out of plane atoms. A knowledge of the symmetry and shape in momentum space of the superconducting order parameter is essential for constructing the correct model of the pairing mechanism. A number of different scenarios have been proposed to explain the mechanism of the superconductivity in this system [16,1...

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